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The key difference between ale and lager is in fermentation; lager is fermented at a much lower temperature than ale and uses a different type of yeast.

The primary fermentation period for lager takes at least twice as long as for ale; this time is furthermore compounded by weeks or months of lagering. The term ‘lagering’ means the storage of beer for long periods of time in cool conditions; originally this would have been in caves, which was a common practice throughout the medieval period. Now, lager is stored in temperature-controlled fermentation and conditioning tanks.

As the low-temperature fermentation (which can take place at temperatures as cool as 0–5°C/32–41°F) allows diacetyl, a natural by-product of fermentation, to remain free in the fermenting beer, the fermentation temperature may briefly be raised near the end of the primary fermentation to allow the consumption of this chemical. This is called the diacetyl rest.

Before the introduction of refrigeration, this reliance on lower temperatures separated Europe into ‘lager’ and ‘ale’ spheres, with warmer countries generally producing ales and colder ones producing lagers. Difficulties in temperature control also create a disincentive for microbrewers to produce lagers because of the huge capital costs of the equipment required.

One exception to the rule of low-temperature lager brewing is found in a beer style known as steam beer, or California Common. The strain of yeast used in steam beer had originated in Germany and was brought over to breweries in the central and eastern states of America. In the 1840s and 50s the same strain was used in breweries in California on the west coast. The higher ambient temperatures in that region caused brewers to favour shallower fermenters in order to better control fermentation; over several generations, evolutionary pressure led to the emergence of a lager strain which produced the best beer at temperatures of 18–20°C (64–68°F).

The choice of lager beer’s grains and hops is, in principle, the same as for ale, despite the nomenclature ‘lager malt’ sometimes encountered in the United Kingdom.

The majority of lager in worldwide production is light in colour and usually represents the helles, pale lager or Pilsner styles. The flavour of these lighter lagers is usually mild and the producers often recommend that the beers be served refrigerated. However, the examples of lager beers produced worldwide vary greatly in flavour, colour and composition.

Most lagers are brewed in the continental style, originating in continental Europe, and consequently follow central european recipe formulations: the grist is composed mostly of Pilsner malt, Vienna malt or Munich malt, with caramel malts added to improve sweetness and head retention, and others added only for colour. The selection of hops is usually made from noble hops such as Saaz, Hallertau, Tettnanger, Strisselspalt or Lubelski.

Lagers often also feature large proportions of adjuncts, usually rice or corn. Adjuncts entered American brewing as a means of thinning out the body of American beers, balancing the large quantities of protein introduced by six-row barley. However, adjuncts are now often used in beer making to introduce a large quantity of sugar, and thereby increase ABV, at a lower price than a formulation using an all-malt grain bill.

In colour, helles and pale lager represent the lightest lagers; the darkest are Baltic porters. Darker German lagers are often referred to as dunkel lagers (dunkel meaning dark in German).